Sequestration of carbon by the oceans and its consequences

The oceans and seas of the world, from which is harvested a third of the protein that people eat, also serve humanity by absorbing a third of the carbon dioxide that humanity produces. The oceans have various ways of doing this and we need the oceans to keep on absorbing carbon dioxide in order to prevent atmospheric carbon dioxide rising even faster, otherwise our climate is likely to change too quickly for us to manage.

The land, like the oceans, offers us ways of storing carbon. We can use trees as carbon stores and do so. We could use them more by planting more trees and more forests, thereby using trees to help us sequestrate carbon dioxide. Perhaps replanting forests across the world will only have, in climate change terms, positive outcomes and that we should encourage that as much as we can.

However, we cannot simply treat the oceans in the same way as we might treat the land. When oceans sequestrate carbon dioxide they change chemically and those chemical changes have adverse effects, unlike the planting of trees on land.  

We should understand the ways in which the oceans store carbon. One process involves plankton. Plankton take up carbon dioxide in the air and convert it into biomass, which then sinks to the bottom of the sea with the carbon. Various attempts are being made to ascertain whether plankton can be artificially encouraged to sequester more carbon than they do, but it seems that the more research scientists carry out the more they know about how little they understand the way to stimulate plankton growth. There is a fear, as yet not proven, that as the oceans warm plankton will not be able to reproduce as quickly as they do now, thus not sequestering the amounts of carbon dioxide that they do at present

Another way that the oceans take up carbon dioxide is that some of the carbon dioxide in the air dissolves in sea water. It is not in any way scientifically controversial that dissolving more and more carbon dioxide in sea water makes the sea water more acidic. At one time it was feared that increasing storms and hurricanes (a possible consequence of climate change) would reduce the ability of the oceans to take up carbon dioxide, but there has been no evidence to support this theory.

It seems that the natural chemical process of carbon dioxide being dissolved in sea water will continue as the amount of atmospheric carbon dioxide remains high, and more carbon dioxide will be dissolved in sea water as concentrations increase. This may appear to be a good thing, but it is a bit like sweeping the dirt under the rug; the carbon dioxide is taken out of the atmosphere (where very high levels will damage us) but put into the seas, where it is create another adverse effect.

As the seas take up more carbon dioxide so the seas become more and more acidic. This acidification makes the survival of coral reefs more difficult. Research indicates that if atmospheric concentrations of carbon dioxide rise to around 750 parts per million (they are now close to 390 ppm) the coral reefs may start to dissolve, according research carried out by the Hebrew University in Jerusalem and the Carnegie Institute in the United States.

Now 750 ppm is a very large number and one that is highly unlikely according to most climate scientists, but the research does show the relatively narrow range of safety. Higher sea temperatures is known to cause coral bleaching – the loss of symbiotic algae with which the coral reefs live. Apparently at 560 ppm (a figure at the high range of climate change science, but not necessarily an impossible one) the combination of bleaching and acidification of sea water will make coral reefs unsustainable in most parts of the world.

It will be a shame to lose the coral reefs and the life that they support but there are even more serious consequences of acidification than loss of coral reefs. Ocean acidification will not just kill off large areas of coral reefs together with their ecosystems and life forms that depend on them. It will also threaten fish – upon which we depend for our protein.

The increasing process of carbon dioxide making oceans more acidic is already beginning to affect marine life. At the bottom of the sea food chain are small shelled molluscs, sometimes known as pteropods. These are a staple food for many fish. The increasingly acidic oceans is dissolving the shells of these molluscs more quickly and interfering with their reproductive ability. At previous pH levels the mollusc shells were sturdy enough to survive. As the seas become acidic the shells become weaker. The molluscs become easy to eat and find it harder to reproduce. There will be a feast for the fish that eat them at first, and then a famine. If we lose the pteropods we will be likely to lose many species of fish. 

I suppose like everything, as wide and as mighty as the ocean is, it is not limitless in the amount of carbon dioxide that it can absorb and the absorption of the oceans of the present levels of carbon dioxide is not without a feed back and consequences. It may be that many of these consequences are unknown and that we are expressing our fears with greater emphasis than the known state of science justifies. However, we should always remember that nothing on our planet is without limit, except perhaps humans ability to act as though nature and science had no rules that humans cannot circumnavigate. 

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